The present invention relates to rechargeable electrochemical energy storage systems, particularly to such systems comprising materials capable of reversibly taking up and releasing lithium ions as a means of storing and supplying electrical energy. More specifically, the invention relates to the synthesis and utilization of nanostructure lithium titanate as the active electrode component material of electrochemical cells capable of exhibiting high specific capacity at high recharge rates.
During the course of development of rechargeable lithium ion cells and the like, numerous materials capable of reversibly accommodating lithium ions have been investigated. Among these, occlusion and intercalation materials, such a carbonaceous and graphitic compounds, and transitions metal oxide spinels, have proved to be particularly well-suited to such applications. However, even while performing significantly well in high capacity recycling electrical storage systems, many of these materials exhibit detrimental properties which detract from the ultimate operation of rechargeable cell devices.
Notable among such shortcomings of active cell materials is the tendency toward physical expansion to a greater or lesser degree upon taking up the mobile ions, e.g., Li+ ions, which form the critical basis for the function of such systems. While the relatively minute size of Li+ ions, as compared to other mobile ion species of varying efficacy, minimizes to a great extent the physical strain upon the electrode material structure, there nonetheless often results from repetitive cycling effects a significant loss of electrochemical cell capacity, whether due to internal disruption of electrical continuity within the structure of the active compound or otherwise.
Extensive investigation into this so-called “electromechanical grinding” effect has revealed certain compounds which exhibit little or no physical expansion or flexing during the recharge cycling lithiation/delithiation process. Particularly notable in this respect is the lithium titanate defect spinel, Li4Ti5O12, reported by T. Ohzuku et al., J. Electrochem. Soc., 142, 1431 (1995), as a “zero-strain” insertion material. The reported test results of the efficacy of this material as an active counter-electrode component in a rechargeable lithium intercalation battery cell showed a remarkable capacity stability over extended cycling which indeed evidenced a low level of physical expansion nominally attributable to zero-strain ion insertion.
However, these promising capacity retention results were achieved at cycling rates of about C/3, i.e., at such low energy density as to require about 3 h to effect a complete charge or discharge operation, thereby allowing sufficient time to all but ensure thorough Li+ ion transfer to or from the active electrode material and provide an encouraging indication of a correspondingly high level of cell specific capacity. Such long charging periods are, unfortunately, impractical and unacceptable in the marketplace. Subsequent testing of similar Li4Ti5O12 material prepared in the manner of Ohzuku, that is, by 10–12 h, 750–800° C. annealing of a mixture of TiO2 and a thermolabile Li precursor, such as LiOH.H2O, at more acceptable cycling rates in excess of about 2C, i.e., at a charging energy density level sufficiently high to yield a full charge in ½ h, revealed that the otherwise promising zero-strain material could not maintain for any reasonable length of time a recharge capacity of more than about 50% of that initially exhibited at low cycling rates.
Attempts to improve recharge capacity of Li4Ti5O12 electrodes at higher cycling rates by reducing the particle size of this incorporated intercalation material were reported by Peramunage et al., J. Electrochem. Soc., 145, 2609 (1998). Such an approach would have appeared to have significant merit, since it could generally be observed that, while the rate at which an ion species, such as Li+, is capable of diffusing into a given host intercalation compound is a function of the molecular structure of such compound, the radial distance the ion must travel to complete intercalation throughout a host particle, i.e., the size of such particle, will determine the overall intercalation capability, or specific capacity, of a mass of the host compound. In such investigations, the authors utilized submicron-sized TiO2 and LiOH or Li2CO3 precursors in the process of Ohzuku (10 h anneal at 800° C.) to reportedly obtain submicron particles of Li4Ti5O12 material and thus substantially reduce the time required to provide thorough intercalation into the material. While some improvement in the recharge capacity of Li battery cells was shown by levels of 100–120 mAh/g at 1C to 1.2C cycle rates, significant capacities achieved at rates above 1.5C remained of borderline practicality.
Further investigations into the problem of high cycle rate recharge capacity of Li4Ti5O12 electrode materials have been undertaken here and have revealed that synthesized Li4Ti5O12 particles continue to enlarge during prolonged annealing, thus leading to detraction from any initial improvement in capacity at high cycling rates which the use of nanostructure precursor material particles might otherwise have provided. Resolution of this problem during the course of the present invention has resulted from the discovery of a process which yields consistent nanostructure Li4Ti5O12 product and provides such materials and electrochemical cells which exhibit remarkable improvement in high cycle rate capacity to the extent of many-fold increases in effective cycle rates. In addition, the invention provides a process for synthesizing such materials which realizes magnitude increases in economies of time and energy.